Structure Control for Fine Tuning Fluorescence Emission from Side

May 23, 2007 - Yield: 0.87 g (87%). mp: 159 °C. 1H NMR (CDCl3) δ ppm: 8.2 (2H, d, Ar), 7.97 (4H, m, Ar), 7.04 (2H, d, Ar), 3.95 (3H, s, −C(O)OCH3)...
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6364

J. Phys. Chem. B 2007, 111, 6364-6373

Structure Control for Fine Tuning Fluorescence Emission from Side-Chain Azobenzene Polymers P. Smitha and S. K. Asha* Polymer Research Group, Chemical Sciences and Technology DiVision, National Institute for Interdisciplinary Science and Technology, (Formerly: Regional Research Laboratory), ThiruVananthapuram - 695019, Kerala, India ReceiVed: February 15, 2007; In Final Form: April 10, 2007

New fluorescent azobenzene dyes and side-chain polymers have been synthesized and characterized and their photophysical properties studied. A series of azobenzene dyes having different fluorophores such as phenol (S1), phenylphenol (S2) and naphthol (S3) incorporated in them were synthesized. S2 had unusually high fluorescence with a quantum yield of φf ) 0.2 recorded in dichloromethane (DCM), whereas S1 and S3 were found to be weakly fluorescent. The azobenzene dyes were converted into methacrylate monomers having short ethyleneoxy spacers and then free radically polymerized. Phenylphenol-based azobenzene polymer (P2) continued to show fluorescence, whereas fluorescence was completely quenched in the case of phenol (P1)and naphthol (P3)-based polymers. Phenylphenol, though twisted in the ground state is known to have a more planar geometry in the excited statesa factor that enables it to retain its fluorescence behavior even when it is incorporated as part of an azobenzene unit. In contrast, naphthol, which is a better fluorophore compared to phenylphenol, loses much of its emissive behavior upon coupling to the azobenzene unit. The extent of trans to cis photoisomerization in solution was very low (∼17%) for P2 after 30 min of continuous irradiation using 365 nm light, in contrast to ∼ 40% for P1 under identical conditions. This is attributed to the steric repulsion brought about by the bulky phenylphenol units that restrict rotation. A 2-fold enhancement in fluorescence emission was observed for P2 upon irradiation by UV light at 360 nm, which relaxed to the original intensity in about 7 day’s time. The higher emission of the cis azobenzenes is generally attributed to an inhibition of photoinduced electron transfer (PET) mechanism. The emission of P2 showed a concentration dependence which increased initially and then decreased in intensity with the formation of a new red-shifted peak at higher concentration due to aggregation. Irradiation of the fluorescence quenched highly concentrated (1 × 10-3 M) sample of P2 showed an enhancement in emission from aggregates at 532 nm.

Introduction Azobenzene and its derivatives are well-known photochromic compounds, as it can undergo photoinduced trans-cis isomerization and azobenzene-containing polymers are potentially useful materials for various optical and photonic applications. However, in general, it is a known fact that azobenzene molecules do not fluoresce in solution with appreciable quantum yield due to their ability to undergo trans-cis isomerization. The first observation of the n-π* (S1 state) luminescence of trans azobenzene was by Struve in 1977 in the 600-800 nm region.1 Later Morgante and Struve studied the fluorescence of trans azobenzene derivatives from the π-π* (S2) state by picosecond laser and reported the λmax at 385 nm with quantum yields in the range 10-5.2,3 Recently, a few reports have appeared on fluorescence emission from self-assembled bilayer aggregates and dendrimers which contain the azobenzene moiety.4-8 There are also reports of metalation of azo group in the trans configuration which showed weak luminescence of the order of 10-4, in contrast to the uncomplexed azobenzene reference which was non luminescent.9 A highly intense fluorescence with quantum yield in the range of 0.2-0.7 (highest known so far) was reported very recently where boron-substituted azobenzene showed strong green fluorescence upon irradiation by 360 nm * Corresponding author. E-mail: [email protected]. Fax: 0091471-2491712.

light.10 These reports of azobenzene emission from bilayers and aggregated species are mostly observed in the 550-600 nm region. Han et. al showed photoinduced formation of blue fluorescent aggregates in the 460-500 nm region for simple azobenzene derivatives with different alkyl chain lengths whose isolated molecules in solution are non fluorescent at ambient temperature.11,12 This photoinduced fluorescence enhancement has been attributed to light driven self-assembly of cis azobenzenes. In contrast to the above reports, Zacharias et al. showed that coupling fluorophores such as 1-aminonaphthalene and 2-amino benzoic acid to azobenzene systems can induce fluorescence in the 400 nm region which showed a 6-fold enhancement in luminescence upon irradiation.13 The origin of this enhanced luminescence was explained as the result of nonplanar geometry in the cis isomer, which obstructs effective conjugation of the lone pair of electrons on the nitrogen with the π electrons of the fluorophore, thereby inhibiting photoinduced electron transfer (PET). The versatility of azobenzene derivatives as promising systems for various applications could be further extended if they emitted fluorescence. Attempts to fine-tune the fluorescence properties of azobenzene derivatives in the literature have mostly concentrated on small molecules only. There are no reports in the literature of intense fluorescence exhibited by azobenzene based polymerssmain chain or side chainsat room temperature

10.1021/jp071303j CCC: $37.00 © 2007 American Chemical Society Published on Web 05/23/2007

Side-Chain Azobenzene Polymers in the trans state. There are reports of fluorescence from poly(p-phenylenevinylene) (PPV) derivatives substituted with azobenzene side chains, but the emission in these cases is entirely due to the PPV backbone.14,15 There is also one report of poly(pphenylene) (PPP) unit in conjugation with azobenzene units in the main chain which showed weak fluorescence expected to be originating from the terphenyl units in the main chain.16 Fluorescence in polymers is an important tool that can be used to study molecular dynamics, as it reflects the energy levels of the microstructure of the polymers. The development of freely soluble azobenzene polymers that can show room-temperature fluorescence is interesting not only as new materials of fundamental scientific interest but also as potential candidates for sensor and other similar applications. Previously we had reported the hyperpolarizability tensor β determined by Hyper Rayleigh Scattering (HRS) measurement for a series of AB type azobenzene monomers having carboxyl (COOH) and hydroxyl (OH) as the acceptor and donor units respectively at the ends and incorporating different types of phenols like simple phenol, phenylphenol, naphthol, etc.17 The naphthol and phenylphenol based azobenzene monomers showed the highest β values in comparison with the simple phenol based monomer. This study showed the effect of minor structural variations on molecular property like the hyperpolarizability tensor. Although main-chain NLO polyesters based on these azobenzene monomers were synthesized, they had the problem of insolubility which prevented further studies of the polymers in solution. In the present study, the azobenzene dyes based on naphthol and phenylphenol having high β values, along with the simple phenol-based one, were introduced as side chains in a methacrylate polymer which made them completely soluble in most of the common organic solvents. The aim of this work is to study the role of molecular geometry in relation to fluorescence behavior in a series of azobenzene molecules and polymers having phenol, phenylphenol and naphthol fluorophores incorporated in them. Here we report, for the first time, the relatively high quantum yield of emission φf ) 0.2 (in contrast to the generally reported values in the range of 10-5)18-20 at 400 nm in an azobenzene dye in dichloromethane, which upon polymerization into a sidechain azobenzene methacrylate polymer continued to fluoresce with a reduced quantum yield of emission φf ) 0.02. The emission behavior of non-irradiated and irradiated (using 360 nm light) solutions of this novel polymer in dilute and concentrated forms were systematically studied. Aggregation tendency of azobenzene systems were studied using variable concentration fluorescence measurements. The influence of trans-cis isomerization and aggregate formation on the fluorescence emission was also studied. The study throws light on the importance of understanding the structure-property relationship in fine-tuning properties while designing materials. For instance, phenylphenol and naphthol are well-known fluorophores; however, coupling of these with azobenzene units resulted in emission from one while the other was almost nonemissive. The molecular geometry factors like coplanarity and conformation in the ground and excited-state plays an important role in deciding the emission properties of the bulk material. Experimental Section Materials. 4-Amino benzoic acid, 4-phenylphenol, and methacryloyl chloride were purchased from Aldrich Chemicals and were used as such. Phenol, R-naphthol, sodium nitrite, potassium hydroxide, potassium carbonate, and potassium iodide were purchase from S.D. Fine Chemicals Ltd. Dimethyl

J. Phys. Chem. B, Vol. 111, No. 23, 2007 6365 formamide (DMF), methanol, tetrahydrofuran, triethyl amine, 2-chloroethanol, and azobisisobutyronitrile (AIBN), were purchased locally and purified using standard procedures. Instrumentation. 1H and 13C NMR spectra of intermediates, monomers, and polymers were recorded using 300-MHz Brucker NMR spectrophotometer in deuterated chloroform containing small amount of tetra methyl silane (TMS) as internal standard. For 13C NMR experiments, the carbon atom in CDCl3 was taken as 77 ppm and all other peaks were assigned with respect to it. The purity of the compounds was determined by JEOL JSM600 fast atom bombardment (FAB) high-resolution mass spectrometry. The purity of the monomer and intermediate compounds were further confirmed by SEC in THF using polystyrene standards. The molecular weights of the polymers were determined by GPC in THF using polystyrene standards for the calibration. Waters 510 Pump connected through three series of Styragel HR columns (HT-3, HT-4, and HT-5) and Waters 410 Differential Refractometer was used for analyzing the samples. The flow rate of the THF was maintained as 1 mL throughout the experiments and 0.2 wt % of (2 mg in 1 mL) the polymer was filtered and injected for recording the GPC chromatograms at 30 °C. Infrared spectra of the polymers were recorded using a Perkin-Elmer, Spectrum one spectrophotometer in the range of 400-4000 cm-1. UV-Vis spectra were recorded using Perkin-Elmer Lambda 35 UV-vis Spectrometer. The emission studies were performed by a SPEX Fluorolog F112X spectrofluorimeter. For recording solid-state photo physical properties, thin films were cast from dichloromethane (DCM) solution on a glass plate and allowed to dry for 24 h prior to measurements. The film thicknesses were varied by changing the concentration of the solution. The fluorescence quantum yields of the azobenzene derivatives and polymers were determined in DCM using quinine sulfate in 0.1M H2SO4 (φ) 0.546)as the standard. The optical density at λmax was maintained at 0.1 ( 0.05 to avoid reabsorption artifacts. The quantum yield was calculated using the equation.21

φs ) φr[FsAr/FrAs](nr/ns)2 where φs is the fluorescence quantum yield of the sample, F is the area of the emission peak, n is the refractive index of solution, and A is the absorbance of the solution at the exciting wavelength. The subscripts r and s denote reference and sample, respectively. Thermogravimetric analysis (TGA) was performed using a TGA-50 Shimadzu Thermogravimetric Analyzer. Samples were run from 40 to 400 °C with a heating rate of 10 °C/min under nitrogen. The thermal and phase behavior of the polymers were analyzed using a hot stage polarized light microscope (Leitz1350 heating stage coupled with PLM) as well as a DSC-PerkinElmer Pyris 6 Differential Scanning Calorimeter (DSC) at a heating rate of 10 °C/min under nitrogen atmosphere. Typically, 2-3 mg of samples was placed in an aluminum pan, sealed properly and scanned from 10 to 250 °C. The instrument was calibrated with indium standards before measurements. Irradiation measurements were carried out in dichloromethane with a 200 W high-pressure mercury lamp in combination with a 360 nm Oriel bandpass filter. The absorbance was recorded at room temperature. The samples were subsequently irradiated, and the absorbance as well as fluorescence were monitored. The backward transition from cis to trans was followed after keeping the sample in dark for different periods of time and monitoring their UV absorption as well as fluorescence. Energy-minimized conformations in the ground state were obtained by AM1

6366 J. Phys. Chem. B, Vol. 111, No. 23, 2007 calculations carried out using Titan Version 1 from Wavefunction Inc; 18401, Von Karman Suite 370, Irvine, CA 92612. Synthesis of 4-(4-Hydroxy-phenylazo)-benzoic Acid Methyl Ester (S1). The hydroxy azobenzoic acids were synthesized as described in our earlier report.17 Esterification of the azobenzoic acid was carried out using standard procedure. 4-[(4-Hydroxyphenyl)azo]-benzoic acid (1 g, 4.1 mmol) was dissolved in 10 mL methanol with catalytic amount of concentrated sulfuric acid and refluxed for 3 h. After distilling off the excess solvent, the crude product was poured into water and neutralized with base. The precipitate was then filtered and dried. The crude dye was further purified by column chromatography on silica gel (60-120 mesh) using hexane-ethyl acetate mixture (v/v 70/30). Yield: 0.87 g (87%). mp: 159 °C. 1H NMR (CDCl3) δ ppm: 8.2 (2H, d, Ar), 7.97 (4H, m, Ar), 7.04 (2H, d, Ar), 3.95 (3H, s, -C(O)OCH3). 13C NMR (CDCl3) δ ppm: 52.3, 115.9, 122.3, 125.2, 130.6, 131.2, 146.9, 155.3, 162.7, 166.6. FTIR (KBr) (cm-1): 3401, 1716, 1599, 1498, 1402, 1238, 1254, 1188, 1139, 1106, 859, 839, 820, 776, 724, 697, 669, 625, 549. FAB- HRMS m+1: 257.08. Similarly, (1 g, 3.1 mmol) of 4-[(4-hydroxybiphenyl-4yl)azo]-benzoic acid and (1 g, 3.4 mmol) of 4-[(4-hydroxynaphthen-1-yl)azo]-benzoic acid were used to obtain the esters 4-(4′hydroxy-biphenyl-4-ylazo)-benzoic acid methyl ester (S2) and 4-(4-hydroxy-naphthalen-1-ylazo)-benzoic acid methyl ester (S3) respectively. 4-(4′-Hydroxy-biphenyl-4-ylazo)-benzoic Acid Methyl Ester (S2). Yield: 0.81 g (78%). mp: 135 °C. 1H NMR (CDCl3) δ ppm: 8.2 (2H, d, Ar), 7.9 (2H, m, Ar), 7.6 (2H, d, Ar), 7.57.1 (4H, m, Ar) 6.8 (2H, d, Ar), 3.89 (3H, s, -C(O)OCH3). 13C NMR (CDCl ) δ ppm: 52.4, 115.7, 122.1, 126.6, 128.2, 3 128.9, 129.8, 130.9, 132.9, 140.8, 151.6, 155.3, 157.4, 166.0. FTIR (KBr) (cm-1): 3368, 1720, 1597, 1427, 1273, 1112, 1015, 963, 861, 831, 689, 635, 593, 562. FAB- HRMS m: 332.31. 4-(4-Hydroxy-naphthalen-1-ylazo)-benzoic Acid Methyl Ester (S3). Yield: 0.87 g (83%). mp: 192 °C. 1H NMR (CDCl3) δ ppm: 8.2-8.1 (3H, m, Ar), 7.9-7.8 (3H, m, Ar), 7.6 (1H, m, Ar), 7.4 (2H, t, Ar), 7.1 (1H, d, Ar), 3.9 (3H, s, -C(O)OCH3). 13C NMR (CDCl3) δ ppm: 52.3, 108.5, 116.3, 121.3, 122.6, 125.8, 125.9, 126.3, 127.8, 130.6, 131.3, 132.9, 141.7, 155.7, 159.3, 166.7. FTIR (KBr) (cm-1): 3407, 1717, 1597, 1575, 1506, 1429, 1390, 1319, 1276, 1248, 1092, 1018, 862, 812, 757, 694, 579. FAB- HRMS m+1: 307.01. Synthesis of 4-[-(2-Hydroxy-ethoxy)-phenylazo]-benzoic Acid Methyl Ester (ACE1). 4-(4-Hydroxy-phenylazo)-benzoic acid methyl ester (S1) (2.50 g, 9.7mmol), anhydrous potassium carbonate (2.02 g, 14.6mmol), catalytic amount of KI, and 2-chloroethanol (0.79 g, 9.7mmol) were dissolved in 10 mL DMF. The mixture was heated at 80 °C for 48 h under nitrogen. The resulting solution was cooled to room temperature and poured into water. The product was filtered, washed with water and dried, and purified by column chromatography using hexane-ethyl acetate mixture (v/v 90/10). Yield: 2.5 g (86%). mp: 104 °C. 1H NMR (CDCl3) δ ppm: 8.2 (2H, d, Ar), 7.9 (4H, m, Ar), 7.1 (2H, d, Ar), 4.5 (2H, t, -OCH2-), 4.0 (2H, t, -CH2O-), 3.9 (3H, s, -C(O)OCH3). 13C NMR (CDCl ) δ ppm: 55.6, 61.4, 66.8, 114.3, 122.4, 125.2, 3 130.7, 131.2, 144.3, 155.3, 159.7, 166.6. FTIR (KBr) (cm-1): 3396, 1714, 1687, 1604, 1583, 1498, 1454, 1402, 1270, 1144, 1026, 861, 837, 774, 694, 543. FAB- HRMS m+1: 301.40. Similarly, (1 g, 2.7 mmol) of S2 and (1 g, 2.9 mmol) of S3 were used to obtain the esters 4-[-(2-Hydroxy-ethoxy)-biphenylazo]-benzoic acid methyl ester (ACE2) and 4-[-(2-Hydroxy-

Smitha and Asha ethoxy)-naphthalen-1-ylazo]-benzoic acid methyl ester (ACE3) respectively. 4-[-(2-Hydroxy-ethoxy)-biphenylazo]-benzoic Acid Methyl Ester (ACE2). Yield: 0.79 g (70%). mp: 140 °C. 1H NMR (CDCl3) δ ppm: 8.2 (2H, d, Ar), 7.9-7.8 (4H, m, Ar), 7.6 (2H, d, Ar), 7.5 (2H, m, Ar), 7.3 (2H, m, Ar), 4.4 (2H, t, -OCH2), 4.0 (2H, t, -CH2O-), 3.9 (3H, s, -C(O)OCH3). 13C NMR (CDCl3) δ ppm: 52.3, 61.0, 71.9, 115.6, 122.1, 126.7, 128.3, 128.9, 130.9, 132.9, 140.6, 151.6, 155.2, 157.0, 166.6. FTIR (KBr) (cm-1): 3362, 1726, 1605, 1459, 1484, 1429, 1273, 1218, 1152, 1114, 1018, 831, 757, 686, 540. FAB- HRMS m+1: 377.13. 4-[-(2-Hydroxy-ethoxy)-naphthalen-1-ylazo]-benzoic Acid Methyl Ester (ACE3). Yield: 0.87 g (76%). mp: 113 °C. 1H NMR (CDCl3) δ ppm: 8.2, 8.1 (3H, m, Ar), 7.9 (2H, m, Ar), 7.6, 7.5 (2H, m, Ar), 7.3 (2H, d, Ar), 6.8 (1H, m, Ar), 4.4 (2H, t, -OCH2-), 4.2 (2H, t, -CH2O-) 3.9 (3H, s, -C(O)OCH3). 13C NMR (CDCl ) δ ppm: 52.5, 61.7, 67.1, 105.3, 113.8, 122.4, 3 122.8, 125.8, 126.2, 126.9, 128.1, 130.9, 131.3, 133, 141.9, 157.0, 159.3, 166.7. FTIR (KBr) (cm-1): 3401, 1720, 1599, 1578, 1509, 1465, 1435, 1391, 1322, 1248, 1276, 1248, 1193, 1135, 1039, 1017, 905, 858, 809, 760, 694. FAB- HRMS m+1: 351.37 Synthesis of 4-{4-[2-(2-Methyl-acryloyloxy)-ethoxy]-phenylazo}-benzoic Acid Methyl Ester (M1): Methacryloyl chloride solution (0.69 g, 6.6 mmol) in tetrahydrofuran (2 mL) was added very slowly to a THF (8 mL) solution of ACE1 (2 g, 6.6mmol) and triethyl amine (0.66 g, 6.6mmol) kept at 0 °C. The solution was stirred for 1 h at 0 °C and then at room temperature for 6h. The salt formed was filtered and removed. The filtrate was then concentrated and the monomer was precipitated in water, filtered, and dried. The crude product was further purified by column chromatography using hexane-ethyl acetate mixture (v/v 95/5). Yield: 1.6 g (65%). mp: 87 °C. 1H NMR (CDCl3) δ ppm: 8.1 (2H, d, Ar), 7.9 (4H, m, Ar), 6.9 (2H, d, Ar), 6.1 (1H, s, -CdCH2), 5.5 (1H, s, -CdCH2), 4.5 (2H, t, -OCH2-), 4.4 (2H, t, -CH2O-), 3.9 (3H, s, -C(O)OCH3), 1.9 (3H, s, CH3). 13C NMR (CDCl3) δ ppm: 18.6, 51.9, 62.8, 66.2, 114.9, 120.3, 122.3, 125.6, 130.7, 131.3, 135.9, 147.2, 155.2, 161.5, 166.6, 167.2. FTIR (KBr) (cm-1): 3428, 1716, 1599, 1498, 1451, 1405, 1273, 1254, 1163, 1138, 1105, 1026, 861, 837, 773, 694, 540. FAB- HRMS m+1: 369.38. Similarly, 1 g and 2.7 mmol of ACE2 and 1 g and 2.9 mmol of ACE3 were used to obtain the monomers 4-{4-[2-(2-methylacryloyloxy)-ethoxy}-biphenylazo} benzoic acid methyl ester (M2) and 4-{4-[2-(2-methyl-acryloyloxy)-ethoxy]-naphthalen1-ylazo} benzoic acid methyl ester (M3) respectively. 4-{4-[2-(2-Methyl-acryloyloxy)-ethoxy}-biphenylazo}benzoic Acid Methyl Ester (M2). Yield: 0.69 g (59%). mp: 99 °C. 1H NMR (CDCl3) δ ppm: 7.5 (2H, m, Ar), 7.4-7.3 (8H, m, Ar), 6.9 (2H, d, Ar), 6.1 (1H, s, -CdCH2), 5.5 (1H, s, -CdCH2), 4.4 (2H, t, -OCH2-), 4.2 (2H, t, -CH2O-), 3.9 (3H, s, -C(O)OCH3), 1.9 (3H, s, CH3). 13C NMR (CDCl3) δ ppm: 18.3, 52.3, 63.2, 66.1, 114.9, 115.3, 121.9, 123.5, 126.8, 127.4, 128.3, 128.8, 130.2, 134.3, 136.1, 140.7, 151.6, 156.4, 158.2. 166.0, 167.2 FTIR (KBr) (cm-1): 3291, 1712, 1606, 1487, 1257, 1056, 835. FAB- HRMS m+1: 445.49. 4-{4-[2-(2-Methyl-acryloyloxy)-ethoxy]-naphthalen-1-ylazo}benzoic Acid Methyl Ester (M3). Yield: 0.72 g (60%). mp: 105 °C. 1H NMR (CDCl3) δ ppm: 8.92 (2H, d, Ar), 8.28 (2H, d, Ar), 8.16 (H, d, Ar), 7.98-7.92 (2H, m, Ar), 7.6 (2H, d, Ar), 6.88 (1H, d, Ar), 6.1 (H, s, -CdCH2), 5.5 (H, s, -Cd CH2), 4.5 (2H, t, -OCH2-), 4.4 (2H, t, -CH2O-), 4.0 (3H, s, -C(O)OCH3), 1.9 (3H, s, CH3). 13C NMR (CDCl3) δ ppm:

Side-Chain Azobenzene Polymers

J. Phys. Chem. B, Vol. 111, No. 23, 2007 6367

SCHEME 1: Synthesis of Monomersa

(a) K2CO3/KI; DMF/80 °C; Cl-CH2-CH2-OH. (b) CH2dC(CH3)-C(O)Cl; TEA/THF, 0 °C.

18.3, 55.9, 62.4, 62.8, 103.7, 117.3, 122.2, 123, 125.2, 125.9, 126.1, 127.8, 129.3, 130.7, 132.4, 136.1, 143.3, 157.0, 159.2, 166.0, 167.2. FTIR (KBr) (cm-1): 3406, 1720, 1599, 1577, 1509, 1459, 1388, 1317, 1275, 1246, 1163, 1092, 1017, 765. FAB- HRMS m: 418.34. Free-Radical Polymerization of Poly 4-{4-[2-(2-methylcryloyloxy)-ethoxy]phenylazo}-benzoic Acid Methyl Ester P1. Free radical polymerization was carried out in THF using AIBN as initiator at 60 °C. M1 (0.36 g, 0.98mmol) and AIBN (0.0184 g, 0.05mmol) were dissolved in THF (2 mL). The reaction mixture was purged with nitrogen for 10 min. The polymerization was carried out by stirring the contents at 60 °C for 24 h. The slightly viscous liquid was cooled and precipitated into methanol. The polymer was purified by filtering and redissolving in tetrahydrofuran followed by repeated reprecipitation into methanol to remove the monomers followed by drying in vacuum at 60 °C for 24 h. Yield: 0.23 g (64%). 1H NMR (CDCl ) δ ppm: 8.1-6.8 (m, Ar), 7.9-7.8 (m, Ar), 3 4.3-3.8 (-OCH2-, OCH3), 2.2-0.8 (aliphatic). 13C NMR (CDCl3) δ ppm: 176.8, 166.7, 165.8, 162.9, 161.4, 155.3, 147.3, 130.7, 125.4, 122.6 114.9, 65.7, 63.4, 55.8, 52.5, 44.9, 29.9, 28.9, 19.1, 17.4. FTIR (KBr) (cm-1): 3406, 1722, 1600, 1500, 1436, 1405, 1279, 1254, 1174, 1140, 1111, 1064, 960, 922, 861, 834, 756, 694, 661, 546. Similarly, (0.20 g, 0.45mmol) M2 and (0.25 g, 0.59mmol) M3 were subjected to free radical polymerization under identical conditions to obtain the polymers Poly 4-{4-[2-(2-methylacryloyloxy)-ethoxy}-biphenylazo}benzoic acid methyl ester (P2) and Poly 4-{4-[2-(2-methyl-acryloyloxy)-ethoxy]-naphthalen-1-ylazo}benzoic acid methyl ester (P3) respectively. Poly 4-{4-[2-(2-Methyl-acryloyloxy)-ethoxy}-biphenylazo}benzoic Acid methyl ester (P2). Yield: 0.09 g (45%). 1H NMR (CDCl3) δ ppm: 7.6-6.8 (m, Ar), 4.3-3.9 (-OCH2-, OCH3), 2.0-0.8 (aliphatic). 13C NMR (CDCl3) δ ppm: 166.4, 155.2, 130.6, 128.8, 126.7, 122.8, 115.3, 111.4, 108.5, 52.3, 29.7. FTIR (KBr) (cm-1): 3434, 1720, 1602, 1514, 1432, 1407, 1276, 1169, 1111, 1018, 862, 820, 760, 697, 464. Poly 4-{4-[2-(2-Methyl-acryloyloxy)-ethoxy]-naphthalen1-ylazo}-benzoic Acid Methyl Ester (P3). Yield: 0.15 g (60%). 1H NMR (CDCl3) δ ppm: 8.9-6.5 (m, Ar), 4.3-3.8 (-OCH2-, OCH3), 1.9-0.9(aliphatic). 13C NMR (CDCl3) δ ppm: 179.4, 177.6, 172.6, 167.5, 130.6, 122.7, 119.4, 116.9, 115.1, 114.6, 113.6, 112.3, 111.8, 110.3, 109.1, 108.7, 104.2, 103.7, 55.9, 29.9. FTIR (KBr) (cm-1): 3412, 1720, 1599, 1578, 1405, 1319, 1273, 1248, 1218, 1095, 1018, 823, 771, 463.

Figure 1. 1H NMR spectra of monomers S1, S2, and S3.

Results and Discussion Synthesis and Characterization. The synthesis of the sidechain methacrylate monomers M1-M3 are shown in Scheme 1. The detailed procedure for the synthesis of the diazo dyes is published elsewhere.17 The diazo dyes were first converted to the corresponding methyl esters (S1-S3) followed by coupling of the phenolic hydroxyl group with 2-chloroethanol in presence of base to obtain ACE1-ACE3. The aliphatic hydroxyl group was then converted to the methacrylate by coupling with methacryloyl chloride in presence of triethyl amine. The structure of methacrylate monomers were confirmed by 1H and 13C NMR, UV-vis and FTIR spectroscopy. The 1H NMR spectra of S1, S2, and S3 are given in Figure 1, and the various types of protons are labeled by alphabets. The intensity of all the peaks exactly matched with the number of protons, which confirm the formation of the expected molecules. The purity was confirmed by the SEC plots of the molecules, which showed a single chromatogram. As expected, the elution time of the molecules increases with the decrease in the molecular weights. The samples were subsequently subjected to HRMS analysis and molecular ion peaks were obtained for m or m+1 radical. The NMR, SEC, and HRMS data confirmed the structure and high purity of the synthesized molecules. The normal free radical polymerization of the monomers were carried out by using 5 mol % AIBN as initiator in tetrahydrofuran (THF) as solvent at 60 °C. The viscous polymer solution was cooled and precipitated into methanol. Scheme 2 shows

6368 J. Phys. Chem. B, Vol. 111, No. 23, 2007

Smitha and Asha

SCHEME 2: Structure of Polymers

TABLE 1: Yield and Molecular Weight Data of Polymers polymer

phenolic component

Mw

PDI

TDa (°C)

P1 P2 P3

phenol phenylphenol naphthol

11000 6800 3800

1.78 1.28 1.79

259 287 306

a Temperature represents 10% weight loss in TGA measurements at heating rate of 10 °C/min under nitrogen.

Figure 2. Comparison of the UV-vis for the phenol, phenylphenol, and naphthol azobenzene starting fluorophores (a); dye esters (b) and polymers (c).

TABLE 2: Spectroscopic Data Recorded in Dichloromethane solution (in DCM)

the structure of all the polymers. The GPC chromatograms and the proton NMR data of the monomer and polymers are given in the supplementary data. The proton NMR shows the complete disappearance of the methacrylic double bonds upon polymerization. Table 1 gives the sample name, corresponding phenolic component and molecular weight details. P2 and P3 had low molecular weight compared to P1. But such low values of molecular weight prepared by free radical polymerization are typical for azobenzene containing polymers.22,23 Additionally, the steric hindrance of the bulky side groups also could result in low degree of polymerization in P2 and P3. The polymers were soluble in most of the common organic solvents such as chloroform, toluene and THF. The thermal characterization of the polymers was done using DSC and TGA. Generally azobenzene systems have a thermal stability limit 30 min) led to deviation from the two isosbestic points in the UV-vis spectra. NMR spectra recorded after 150 min of continuous irradiation did not indicate any decomposition and also, the UV-vis spectrum of the 150 min irradiated sample taken after 7 days of incubation in the dark, showed almost complete recovery indicating that no other side reactions have taken place (given as supplementary data). The extent of conversion to the cis isomer after 30 min of irradiation, calculated based on the decrease in intensity of the π-π* peak maxima, was only around 17% for P2, in contrast to ∼40% for P1 under identical conditions. A dilute (1 × 10-6 M) solution of P2 in DCM was exposed to continuous UV light irradiation at 360 nm and the fluorescence spectra showed a 2-fold enhancement in the intensity recorded at 401 nm without any shift in peak wavelength. Figure 7 shows the steady increase in fluorescence intensity which became stable after ∼150 min of irradiation. The

Side-Chain Azobenzene Polymers

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Figure 11. FTIR spectra recorded at room temperature between 1750 and 1650 cm-1 for S2 and P2.

Figure 9. Fluorescence spectra of S2 (a) in DCM as a function of concentration. Insets depict the variation in the total emission area and peak maximum as a function of concentration. Fluorescence spectra of irradiated samples (1 × 10-3 M) of S2 (b) after various time intervals.

Figure 12. Comparison of UV (a, b) and fluorescence (a′, b′) of P2 solution and film.

Figure 10. Schematic representation of the aggregates of P2 and S2 upon photoirradiation.

fluorescence intensity slowly relaxed back to the original unirradiated value after about 7 days of incubation in the dark. The UV-vis spectra also relaxed back to the original value after 7 days of dark incubation. A similar behavior was shown by the analogous small molecule S2 also upon irradiation under identical conditions for 150 min (see supplementary Figure 10). Additional support for the photoenhanced fluorescence of P2 on trans to cis photoisomerization was obtained from a control polymer that was structurally analogous to P2 but without the NdN unit in it.32 The emission spectra of the control polymer under similar conditions of irradiation did not show any variation over a period of 150 min (given as supplementary data). Therefore, the fluorescence enhancement of P2 is associated with the trans to cis isomerization with respect to the NdN bond present in P2. In the literature, the fluorescence enhancement upon cis isomerization has been attributed to inhibition of photoinduced electron transfer (PET).13 Evidence of the existence of PET phenomenon was examined by protonation

experiments. A dilute (1 × 10-6 M) solution of P2 in DCM was treated with a drop of trifluoroacetic acid and emission was recorded before and after treatment with acid. An enhancement of fluorescence by 4 fold was observed due to protonation of the -NdN- bond by the acid (see Supporting Information Figure 12). This is similar to the observation of Zacharias et.al, who also assumed the inhibition of PET in the cis isomer to result in fluorescence enhancement and found a 3-fold enhancement of fluorescence emission upon protonation with acid.13 The cis isomer that is formed upon photoisomerization has a nonplanar structure for the aromatic rings that are connected directly to the azo nitrogen atoms. This nonplanarity across the NdN reduces the effective conjugation of the nitrogen lone pair of electrons with the π electrons of the fluorophore, thereby inhibiting the PET mechanism. Since the extent of conversion to the cis isomer is very low (∼17%), we do not see a very high enhancement in fluorescence upon photoisomerization like those reported by others. Figures 8a and 9a show variable concentration fluorescence measurement carried out for P2 and S2 respectively in dichloromethane at concentrations ranging from 6 × 10-5 to 1 × 10-3 M. The UV-vis spectra did not show any change in either the wavelengths or the shapes of the absorption signals, except for increased intensity, indicating that no ground state intermolecular aggregates are formed. However changes were observed in the respective fluorescence spectra when the concentration was increased. A 1 × 10-6 M solution of P2 had fluorescence λmax centered at 401 nm (Figure 7) whereas 6 × 10-5 M solution

6372 J. Phys. Chem. B, Vol. 111, No. 23, 2007 (first curve in Figure 8a) has the λmax centered at 420 nm. The fluorescence intensity was proportional to concentration from 1 × 10-6 M to 6 × 10-6 M after which fluorescence self-quenching occurred. Upon further increase of concentration the λmax value was further red-shifted to 498 nm for the highest concentration of 1 × 10-3 M with almost complete quenching of fluorescence. The inset in Figure 8a and 9a shows the variation in the total emission area and emission maxima as a function of concentration. The integrated intensity gives a more complete measure of the spectra rather than the peak intensity.33,34 In both cases there is a sudden change in both the area as well as emission maxima value occurring over a small concentration window, with fairly good matching of the point of steep variation. This sudden variation is suggestive of some kind of cooperative effect like aggregation, interchain exciton formation etc.34 In the case of S2, there is the clear evidence of two peaks at higher concentrationsone at lower wavelength corresponding to emission from molecularly dissolved species and the one at higher wavelength (535 nm) originating from aggregated species. The most concentrated solution, i.e., 1 × 10-3 M of both P2 and S2 were exposed to continuous UV irradiation at 360 nm, and their fluorescence spectra were recorded after varying intervals of time. Figures 8b and 9b show the irradiation spectra of P2 and S2, respectively. The 1 × 10-3 M solution of P2 which was almost nonfluorescent due to concentration quenching, starts fluorescing again at a much higher wavelength of 532 nm upon irradiation.35 In the case of S2, the fluorescence was not fully quenched for the 1 × 10-3 M sample but had two relatively sharp peaks centered at 390 and 535 nm with a shoulder at 569 nm. Upon continuous irradiation, the aggregation peak at 535 nm started decreasing while the emission from the molecularly dissolved species at 390 nm started increasing in intensity. A similar observation was reported by Mina Han et. al for their amphiphilic azobenzene molecule which had a biphenyl fluorophore and long alkyl chain whose highly concentrated nonfluorescent sample exhibited two discrete emission peaks centered at 420 and 546 nm after UV irradiation.35 They showed the existence of aggregates using SEM and TEM as well as fluorescence microscopic images. On the basis of these results, we can assume that the fluorescence of the phenylphenol based azobenzene dye ester (S2) and polymer (P2) originates from the trans form at room temperature. There is no aggregate formation in the ground state; however, in the excited state, J-type aggregates are formed at higher concentration as evidenced by the red-shifted emission peak for the high concentrated samples of both S2 and P2. However, the mechanism of aggregate formation is not the same in both cases. For instance, S2 possessing a hydroxyl group can form aggregates due to hydrogen bonding at higher concentration, as evidenced by room-temperature FTIR spectra, which will be discussed later on. The excited-state planar conformation of the biphenyl unit can also favor a J-type aggregate formation. Irradiation using 360 nm UV light results in the photoisomerization of the trans to the cis form which has a rather destabilizing effect upon the S2 aggregates. The photoisomerization destroys the organized aggregated phase of S2 and the chromophores become isolated. Hence, as irradiation time is increased from 30 to 90 min, the aggregate peak at 535 nm reduces in intensity and monomeric fluorescence intensity increases as the number of isolated molecules (both cis and trans) increases. In the case of P2, association due to hydrogen bonding can be ruled out. Although side-chain azobenzene polymers are known to self-organize due to strong

Smitha and Asha π-π interactions leading to interdigitation of the chromophores, such an organization also can be ruled out for P2 since the twisted nature of the phenylphenol unit hinders any close contact. One probable mode of association, occurring especially at higher concentrations is that of a hydrophilic versus hydrophobic interaction, brought about by the ethyleneoxy spacer in P2 linking the azobenzene chromophore to the ‘all-alkyl chain’ backbone. Figure 10 shows a schematic representation of the changes happening to the aggregates of P2 and S2 upon irradiation. It shows that in contrast to S2, P2 can form much more stable aggregates, which remains unchanged by irradiation; whereas the hydrogen bonded aggregates of S2 disrupted upon formation of the bent cis isomers. Assuming that the extent of isomerization does not vary much at higher concentration also, the small extent of cis isomers formed (∼17%) in the highly concentrated solution upon irradiation once again gives rise to a slight enhancement of the aggregation peak at 535 nm by inhibiting the PET mechanism. Proof of the existence of hydrogen bonding in S2 was traced using FTIR spectroscopy. Figure 11 shows the infrared spectra of the carbonyl stretching of S2 and P2. P2 gives a carbonyl absorption peak at 1722 cm-1, whereas S2 shows a carbonyl stretching frequency that is split into two bands at 1721 and 1692 cm-1 respectively. The peak at higher wavenumber corresponds to the free carbonyl stretch and the one at lower wavenumber corresponds to hydrogen-bonded carbonyl stretching frequency.36 In P2 there is no possibility of hydrogen bond formation and therefore, the spectrum shows only one carbonyl absorption peak. On the other hand, S2 clearly shows the evidence of hydrogen bond formation. Additional information regarding the intramolecular conformational changes of the phenylphenol unit enhancing the fluorescence was obtained from solid-state luminescence studies. The general trend of chromophores and polymers that show high fluorescence in solution is to have a completely quenched fluorescence in the solid-state due to processes like intermolecular vibronic interactions which induce nonradiative deactivation.37 Thin films of P2 made by drop casting on glass substrates gave an absorption spectra that was red-shifted by ∼40 nm compared with the solution. Figure 12 compares the fluorescence spectra of P2 in film and in solution in DCM having ∼0.1 OD absorption. The emission spectra of P2 upon excitation at λ320 nm shows a >7-fold enhancement for the film compared with solution. Conformational studies on biphenyl have revealed that a more planar conformation is favored in the solid state.38,39 This planarization can extend the effective conjugation length thereby enhancing the fluorescence.40 Conclusions Fluorescent azobenzene dyes and side-chain polymers comprising fluorophores like phenylphenol and naphthol units have been synthesized. The phenylphenol based azobenzene dye and polymer showed high fluorescence. The molecular geometry of the phenylphenol unit which has a twisted ground state structure and more planar excited-state is the cause of this fluorescence emission in spite of its incorporation into a normally nonfluorescing azobenzene system. The phenylphenol azobenzene polymer exhibited photoinduced enhanced emission in solution upon irradiation. The enhancement of fluorescence emission intensity in dilute solution upon photoirradiation can be explained in terms of the inhibition of photoinduced electrontransfer mechanism. Highly concentrated solutions of the azobenzene phenylphenol dye and polymer showed red-shifted aggregated fluorescence peak. This aggregated emission in-

Side-Chain Azobenzene Polymers creased in intensity following continuous irradiation in the polymer whereas in the dye the intensity of the aggregated emission decreased with simultaneous increase of intensity of molecularly dissolved species. Aggregation is driven by hydrogen bonding in the hydroxyl azobenzene dye molecule which gets disrupted due to the bent cis molecules upon photoirradiation. On the other hand, the aggregation of the highly concentrated polymer solution is brought about by hydrophilichydrophobic interactions and remains stable upon formation of cis isomers during photoirradiation. These observations in the polymer are based on oligomeric species having on an average 12 repeating units. However, it is expected that high molecular weight polymers also will behave similarly under identical conditions. Attempts at building up the molecular weight by adopting other synthetic methodology is currently under progress. In conclusion, we have shown a new design strategy for roomtemperature fluorescing azobenzene molecule and polymer by effectively controlling the molecular geometry of the fluorophore attached to the azolinkage. Acknowledgment. The authors would like to thank the CSIR Task Force Project COR004 and DST Fast Track Scheme for Young Scientists (Project No. SR/FTP/CS-33/2004) for supporting this research. We thank Dr. Suresh Das, RRL-Trivandrum, for isomerization studies. S.P. thanks CSIR-New Delhi, India, for Senior Research Fellowship. Supporting Information Available: SEC plot, 1H NMR and HRMS spectra, UV-vis and fluorescence spectra of monomers, trans to cis photoisomerization spectra, emission spectra of control polymer upon photoirradiation, and TGA plot. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Struve, W. S. Chem. Phys. Lett. 1977, 46, 15. (2) . Morgante, C. G.; Struve, W. S. Chem. Phys. Lett. 1979, 68, 267. (3) Tamai, N.; Miyasaka, H. Chem. ReV. 2000, 100, 1875. (4) Shimomura, M.; Kunitake, T. J. Am. Chem. Soc. 1987, 109, 5175. (5) Constantino, C. J. L.; Aroca, R. F.; Mendonca, C. R.; Mello, S. V.; Balogh, D. T.; Oliveira, O. N. Spectrochim. Acta., Part A 2001, 57, 281. (6) Tsuda, K.; Dol, G. C.; Gensch, T.; Hofkens, J.; Latterini, L.; Weener, J. W.; Meijer, E. W.; De Schryver, F. C. J. Am. Chem. Soc. 2000, 122, 3445. (7) Nithyanandhan, J.; Jayaraman, N.; Davis, R.; Das, S. Chem. Eur. J. 2004, 10, 689.

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